This invention relates to a process for a selective hydrogenation of unsaturated oils, especially edible oils, by which the content in highly unsaturated compounds is strongly reduced with a minimum formation of solid products.
Edible oils essentially consist of esters, mainly glycerides, of various fatty acids, a portion of which is unsaturated and may contain 1, 2, 3 or more double bonds. For example, some vegetable oils, such as soyabean oil, sunflower oil, rapeseed oil, or corn oil, contain compounds having several double bonds (polyenic compounds), for instance, compounds having three double bonds (trienic compounds), and compounds having two double bonds (dienic compounds), together with compounds having only one double bond (monoenic compounds), and with saturated compounds. For example, soyabean oil contains triglycerides of the following acids: about 6 to 10% by weight of linolenic acid (a fatty acid containing 18 carbon atoms and 3 carbon-to-carbon double bonds) about 50% of linoleic acid (a fatty acid containing 18 carbon atoms and 2 double bonds), oleic acid (a fatty acid containing 18 carbon atoms and 1 double bond) and saturated acids (stearic and palmitic acids).
In order to increase the stability of these oils, it is necessary to substantially reduce the content in linolenic acid esters and to partly reduce the content in esters of dienic acids. However, for some uses of these oils, for instance, for their use in foods, e.g., as cooking oil for food, it is important to ensure a certain content in monoenic esters. Reducing the content of polyenic compounds is generally effected by hydrogenation. Yet, in order to avoid or at least partly avoid the formation of solid products and to provide for sufficient monoenic compounds this hydrogenation should be incomplete and selective. In this respect, it is extremely important to limit the formation of saturated compounds, that is, to selectively hydrogenate the polyenic compounds to dienic and monenic compounds, and to limit the formation of trans isomers, which have a melting point higher than the melting point of the cis isomers. Hydrogen treatment usually leads to the formation of these trans isomers, and thus, to the formation of solid hydrogenated oils.
It has already been suggested to carry out a selective hydrogenating treatment of oils in the presence of copper catalysts. However, these catalysts have some disadvantages. For instance, the presence, even of traces, of these catalysts in the hydrogenated products must be avoided because copper induces the oxidation of these products. Moreover, copper catalysts are far less active than nickel catalysts. On the other hand, hydrogenation in the presence of a conventional nickel catalyst is less selective and the amount of solid products resulting from such a hydrogenation is too high.